Polyphenylene ether (PPE) resins and particularly PPO, or poly(2,6-dimethyl-1,4-phenylene ether), have long been known as high temperature thermoplastics. These resins are typically prepared by the oxidative coupling of a monohydric phenol such as 2,6-dimethyl phenol, as taught for example in U.S. Pat. Nos. 3,134,753 and 3,306,874. PPE resins soften or melt only at very high temperatures, normally well above 500.degree. F., which may exceed the upper limit of thermal stability for such materials. The resins also exhibit a very high melt viscosity, and are very difficult to melt process in conventional processing equipment without substantial thermal degradation. Commonly, the resins are blended with lower melt temperature resins to improve their melt processability and to achieve commercially acceptable molding characteristics. For example, polyphenylene ether resins may be blended with styrenic resins as is disclosed in U.S. Pat. Nos. 3,356,761 and 3,383,435 or with polyolefins such as polyethylene as described in U.S. Pat. No. 3,361,851 to achieve improved processability. Blends of poly-2,6-disubstituted phenylene ethers with aromatic polycarbonate resins are somewhat more processable than PPO alone and are craze resistant, as is disclosed in U.S. Pat. No. 3,221,080. These latter blends are generally brittle and require the addition of a third component such as high impact polystyrene to further improve the molding characteristics and to achieve mechanical properties adequate for use in molding applications, as shown in U.S. Pat. No. 3,933,941.
More recently, in U.S. Pat. No. 4,038,543, there were disclosed phenylene ether copolymer resins (PEC) having improved thermal resistance, high temperature oxidative stability and resistance to heat aging. These resins, copolymers of 2,6-dialkyl phenols and 2,3,6-trialkyl phenols containing up to 50 wt% of the trialkyl monomer component, are more stable at the melt processing temperature, but exhibit a high melt viscosity and are also difficult to melt process alone. Blending these copolymers with other resins such as styrenic polymers is therefore necessary to achieve adequate processability and a useful balance of mechanical properties for commercial molding applications.
Although blends of polyphenylene ether resins or phenylene ether copolymer resins with styrenic resins achieves an improvement in processability and when further formulated produces molding resins with a useful balance of mechanical properties, the blends generally exhibit a significantly lowered heat distortion temperature, and the upper use temperature of these blends is considerably less than that of the phenylene ether resin alone. The market place continues to find need for processable thermoplastic molding resins with a good balance of mechanical properties and even higher end-use temperatures, demands which are not satisfactorily met by the presently available resin compositions.